It is believed that methane is produced during the conversion of peat to coal. The conversion is believed to be a result of naturally occurring thermal and biogenic processes. Because of the mutual attraction between the carbonaceous matrix of coal and the methane molecules, a large amount of methane can remain trapped in-situ as gas adhered to the carbonaceous products formed by the thermal and biogenic processes. In addition to methane, lesser amounts of other compounds such as water, nitrogen, carbon dioxide, and heavier hydrocarbons, and sometimes small amounts of other fluids such as argon and oxygen, can be found within the carbonaceous matrix of the formation. The gaseous fluids which are produced from coal formations collectively are often referred to as "coalbed methane." Coalbed methane typically comprises more than about 90 to 95 volume percent methane. The reserves of such coalbed methane in the United States and around the world are huge. Most of these reserves are found in coal beds, but significant reserves may be found in gas shales and other solid carbonaceous subterranean formations believed to result from the action of thermal and biogenic processes on decaying organic matter.
Methane is the primary component of natural gas, a widely-used fuel source. Coalbed methane is now produced from coal seams for use as a fuel. Typically, a wellbore is drilled which penetrates one or more coal seams. The wellbore is utilized to recover coalbed methane from the seam or seams. The pressure difference between a coal seam and the wellbore provides the driving force for flowing coalbed methane to and out the wellbore. Reduction of pressure in the coal seam as coalbed methane is produced increases desorption of methane from the carbonaceous matrix of the formation, but, at the same time, deprives the system of the driving force necessary to flow coalbed methane to the wellbore. Consequently, this method loses its effectiveness over time for producing recoverable coalbed methane reserves. It is generally believed that this method is only capable of economically producing about 35 to 70% of the methane contained in a coal seam.
An improved method for producing coalbed methane is disclosed in U.S. Pat. No. 5,014,785 to Purl, et al. In this process, a methane-desorbing gas such as an inert gas is injected into a solid carbonaceous subterranean formation through at least one injection well, with a methane-containing gas recovered from at least one production well. The desorbing gas, preferably nitrogen, mitigates depletion of pressure within the formation and is believed to desorb methane from the carbonaceous matrix of the formation by decreasing the methane partial pressure within the formation. This method is effective for increasing both the total amount and rate of methane production from a solid carbonaceous subterranean formation such as a coal seam. Present indications are that the rate of methane production can be increased and that the total amount of methane recovered can be increased substantially, possibly up to 80% or more of the methane contained in the formation.
Puri, et al., U.S. Pat. No. 5,014,785, further discloses that air is a suitable source of nitrogen for increasing methane production. However, injecting an oxygen-containing gas, such as air, into a solid carbonaceous subterranean formation, such as a coal seam, to increase production of methane can present problems. Oxygen can cause corrosion and rust formation in well casings and other fluid conduits. Also, injected oxygen-containing gases are potentially flammable. It is desirable to provide an economically attractive method to minimize these potential problems by depleting the oxygen content of air before injecting the oxygen-depleted air into a solid carbonaceous subterranean formation, such as a coal seam, for increasing methane production.
U.S. Pat. No. 5,133,406 to Purl, et al., discloses depleting the oxygen content of air before injecting air into a coal seam by putting air and a source of fuel, such as methane, into a fuel cell power system, generating electricity, and forming a fuel cell exhaust comprising oxygen-depleted air. While this system is advantageous for producing oxygen-depleted air at remote locations, particularly where there is need for generating electricity for on-site needs, there is a need for less expensive methods of producing oxygen-depleted air suitable for use in the production of coalbed methane, particularly where there is not a need for additional on-site electricity.
As used herein, the following terms shall have the following meanings:
(a) "adsorbate" is that portion of a gaseous mixture which is preferentially adsorbed by a bed of adsorptive material during the adsorptive portion of a pressure swing adsorption separator's cycle.
(b) "air" refers to any gaseous mixture containing at least 15 volume percent oxygen and at least 60 volume percent nitrogen. Preferably "air" is the atmospheric mixture of gases found at the well site and contains between about 20 and 22 volume percent oxygen and between about 78 and 80 volume percent nitrogen.
(c) "cleat" or "cleat system" is the natural system of fractures within a solid carbonaceous subterranean formation.
(d) a "coalbed" comprises one or more coal seams in fluid communication.
(e) "formation parting pressure" and "parting pressure" mean the pressure needed to open a formation and propagate an induced fracture through the formation.
(f) "fracture half-length" is the distance, measured along a fracture, from a wellbore to a tip of the fracture.
(g) "preferentially adsorbing", "preferentially adsorbs", and "preferential adsorption" refer to processes that alter the relative proportions of the components of a gaseous fluid. The processes fractionate a mixture of gases by equilibrium separation, kinetic separation, steric separation, and any other process or combinations of processes which within a bed of material would selectively fractionate a mixture of gases into an oxygen-depleted fraction and an oxygen-enriched fraction.
(h) "raffinate" refers to that portion of the gas injected into a bed of adsorptive material which is not preferentially adsorbed by the bed of adsorptive material.
(i) "recovering" means a controlled collection and/or disposition of a gas, such as storing the gas in a tank or distributing the gas through a pipeline. "Recovering" specifically excludes venting the gas into the atmosphere.
(j) "reservoir pressure" means the pressure of a productive formation near a well during shut-in of that well. The reservoir pressure of the formation may change over time as oxygen-depleted effluent is injected into the formation.
(k) "solid carbonaceous subterranean formation" refers to any substantially solid, methane-containing material located below the surface of the earth. It is believed that these solid, methane-containing materials are produced by the thermal and biogenic degradation of organic matter. Solid carbonaceous subterranean formations include but are not limited to coalbeds and other carbonaceous formations such as shales.
(l) "steric separation" occurs in some zeolitic materials where one or more components of a gas mixture are excluded from entering the internal structure of the particles utilized in a bed of such material.
(m) "well spacing" or "spacing" is the straight line distance between the individual wellbores of a production well and an injection well. The distance is measured from where the wellbores intercept the formation of interest.